1. Field of the Invention
The present invention relates to a water method for treating resin in a purge vessel and more particularly to a method for removing unpolymerized gaseous hydrocarbon monomers while deactivating catalyst from granular, low pressure-polymerized, low density ethylene hydrocarbon polymers.
2. Description of the Prior Art
It has long been known that olefins such as ethylene can be polymerized by contacting them under polymerization conditions with a catalyst comprising a transition metal compound, e.g., titanium tetrachloride and a cocatalyst or activator, e.g., an organometallic compound such as triethyl aluminum. Catalysts of this type are generally referred to as Ziegler catalysts.
Low density ethylene polymers (i.e. ethylene polymers having a density of about 0.94 g/cc and lower) have in the past been made commercially by a high pressure (i.e., at pressures of 15,000 psi and higher) homopolymerization of ethylene in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. Recently, low pressure processes for preparing low density ethylene polymers have been developed which have significant advantages as compared to the conventional high pressure process. One such low pressure process is disclosed in commonly assigned, U.S. Pat. No. 4,302,565, the disclosure of which is hereby incorporated herein by reference. Ethylene polymers made by such a low pressure process may be formed into film by known techniques and such film is extremely tough and is useful in packaging applications.
The above-identified patent discloses a low pressure, gas phase process for producing low density ethylene copolymer having a wide density range of about 0.91 to about 0.94 g/cc and a melt flow ratio of from about 22 to about 36 and which have a relatively low residual catalyst content and a relatively high bulk density. The process comprises copolymerizing ethylene with one or more C.sub.3 to C.sub.8 alpha olefin hydrocarbons in the presence of a high activity magnesium-titanium complex catalyst prepared under specific activation conditions with an organo aluminum compound and impregnated in a porous inert carrier material. The copolymers (as applied to these polymers, the term "copolymers" as used herein is also meant to include polymers of ethylene with 2 or more comonomers) thus prepared are copolymers of predominantly (at least about 90 mole percent) ethylene and a minor portion (not more than 10 mole percent) of one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons which should not contain any branching on any of their carbon atoms which is closer than the fourth carbon atom. Examples of such alpha-olefin hydrocarbons are propylene, butene-1, hexene-1,4 -methyl pentene-1 and octene-1.
The catalyst may be prepared by first preparing a precursor from a titanium compound (e.g., TiCl.sub.4), a magnesium compound (e.g., MgCl.sub.2) and an electron donor compound (e.g., tetrahydrofuran) by, for example, dissolving the titanium and magnesium compounds in the electron donor compound and isolating the precursor by crystallization. A porous inert carrier (such as silica) is then impregnated with the precursor such as by dissolving the precursor in the electron donor compound, admixing the support with the dissolved precursor followed by drying to remove the solvent. The resulting impregnated support may be activated by treatment with an activator compound (e.g., triethyl aluminum).
The polymerization process can be conducted by contacting the monomers, in the gas phase, such as in a fluidized bed, with the activated catalyst at a temperature of about 30.degree. C. to 105.degree. C. and a low pressure of up to about 1000 psi (e.g., from about 150 to 350 psi).
The resulting granular polymers may contain gaseous unpolymerized monomers including hydrocarbon monomers. These gaseous monomers should be removed from the granular resin for safety reasons, since there is a danger of explosion if the hydrocarbon monomer concentration becomes excessive in the presence of oxygen. In addition, proper disposal of the hydrocarbon is required in order to meet environmental standards concerning hydrocarbon emissions.
The prior art teaches techniques for removing volatile unpolymerized monomers from polymers of the corresponding monomers. See for example, U.S. Pat. Nos. 4,197,399, 3,594,356, and 3,450,183.
More recently U.S. Pat. No. 4,372,758 issued Feb. 8, 1983 to R. W. Bobst et al and which is assigned to a common assignee discloses, a degassing or purging process for removing unpolymerized gaseous monomers from solid olefin polymers. The purging process generally comprises conveying the solid polymer (e.g., in granular form) to a purge vessel and contacting the polymer in the purge vessel with a countercurrent inert gas purge stream to strip away the monomer gases which are evolved from the polymer.
Unfortunately however in the process for producing polyethylene and polypropylene using Ziegler-Natta catalyst, catalyst and cocatalyst residues in resin entering the purge vessel are not deactivated by countercurrent purging with an inert gas stream as described above. These residues react with air and moisture on exiting the purge vessel and form alcohols, aldehydes, ketones, and alkanes. The alcohols, aldehydes, and ketones formed by reaction with oxygen contribute to resin odor. The alkanes formed by reaction with water require proper disposal in order to meet environmental standards concerning hydrocarbon emissions. In addition, the gaseous hydrocarbon products should be removed from the resin for safety reasons, since there is danger of explosion if the hydrocarbon concentration becomes excessive in the presence of oxygen.
The art has resorted to a moisture treatment of resin prior to exposing the catalyst and cocatalyst residues in the resin to the atmosphere (oxygen) which led to the addition of steam to the inert gas purge stream. The excess moisture required to drive the hydrolysis reaction toward completion was carried out in the vent stream from the top of the purge bin. The presence of moisture was not a concern when the purge bin vent stream was routed to a flare, but presented problems when the vent stream was sent to a monomer recovery unit. Removal of moisture from the purge bin vent stream was required to avoid monomer recovery unit processing problems such as condenser icing and to avoid recycle of moisture with monomer to the reactor which adversely affects catalyst productivity and resin product properties.
One solution for eliminating moisture from the purge bin vent stream was a dual molecular sieve bed drying system that required frequent regeneration with high temperature nitrogen plus a blower to overcome the pressure drop of the purge bin vent stream through the sieve bed. When high levels of moisture addition to the purge bin were required, this solution became unattractive due to limitations on molecular sieve bed size resulting in impractical regeneration frequencies and high regeneration nitrogen supply requirements.
Another solution for eliminating moisture from the purge bin vent stream was the use of two separate bins. One bin was used for dry inert gas purging of residual monomers from the resin with the vent from this bin routed to a monomer recovery unit. The second bin was used for moisture treatment of the resin with the vent from this bin routed to a flare. This solution became commercially unattractive due to the cost and increased space required to either increase the purge bin structure height to accommodate gravity flow of resin between bins or add conveying facilities to transfer resin from one bin to the other bin in a separate structure.
Other techniques for deactivating catalysts residue from polymer resins are disclosed for example in U.S. Pat. No. 4,029,877 issued June 14, 1977; U.S. Pat. No. 4,314,053 issued Feb. 2, 1982 and British patent No. 1,553,565 issued Oct. 3, 1979. These patents disclose the deactivation of the catalyst residues by utilization of water which reacts with the catalyst residues rendering them inactive. Unfortunately however the water treatment disclosed therein has the disadvantage that the monomers present in the polymer resin particles are subject to poisoning and require separate removal steps to remove the water from the monomer.